The present invention relates to tone modifiers for use in thermographic recording materials.
M. Ikeda in 1980 in Photographic Science and Engineering, Volume 24, Number 6, pages 277-280, disclosed a thermodynamic and NMR study on several silver salts of fatty acids (silver behenate, silver stearate, silver palmitate, silver myristate and silver laurate). The results of thermal analyses indicated that the salts exhibit thermotropic liquid crystalline behaviour analogous to those of the alkali metal soaps and that formation in silver behenate of the mesophase, called xe2x80x9csub-waxyxe2x80x9d, was available for development in the case of a commercial dry silver paper. M. Ikeda and Y. Iwata also in 1980 in Photographic Science and Engineering, Volume 24, Number 6, pages 273-276, disclosed a study of the morphology and structure of silver laurate and silver behenate. It was found that these salts undergo phase transitions with increasing temperature. A polarizing microscope was used to determine how the molecular alignment in silver laurate changed with increasing temperature, a super-curd phase (implying crystal phases having different crystal structure) being observed at 109xc2x0 C. and a sub-waxy phase (a mesophase inherent in liquid crystals) at 114xc2x0 C. In the mesophase, formed at a temperature higher than 120xc2x0 C., which generally corresponded to the heat-developable temperature for dry silver paper, the arrangement of Ag atoms was unalterable, but the orientation of the paraffinic chains was random. For silver behenate the long spacing, the distance between Ag atom layers, was found to be unalterable through sub-waxy.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter.
U.S. Pat. No. 3,080,254 specifically discloses a substantially light-insensitive thermographic recording material with a thermosensitive element containing silver behenate, phthalazinone and polyvinyl butyral.
U.S. Pat. No. 3,951,660 discloses a photographic radiation sensitive recording material having therein a radiation sensitive composition and at least one layer containing dispersed in a binding agent a substantially non-light sensitive silver salt, a reducing agent for the non-light-sensitive salt, and a toner compound, the improvement which comprises the toner being a heterocyclic toner compound of the following formula: 
in which X represents O or Nxe2x80x94R5; R1, R2, R3 or R4 represent hydrogen, alkyl, cycloalkyl, alkoxy, alkylthio, hydroxy, dialkylamino or halogen, in addition to which R1 and R2 or R2 and R3 or R3 and R4 can represent the ring members required to complete an anullated aromatic ring, and R5 represents alkyl.
EP 599 369, EP 669 875, EP 669 876 and EP 726 852 disclose in their invention examples substantially light-insensitive thermographic recording materials with a thermosensitive element consisting of silver behenate, 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-5,6,5xe2x80x2,6xe2x80x2-tetrahydroxy-1xe2x80x21xe2x80x2-spiro-bis-indane, polyvinyl butyral, benzo[e][1,3]oxazine-2,4-dione (compound 25 in U.S. Pat. No. 3,951,660) and silicone oil in which the weight ratio of silver behenate to polyvinyl butyral varies between 2:1 and 1:1 and the molar ratio of benzo[e][1,3]oxazine-2,4-dione to silver behenate is about 0.20.
EP-A 752 616 discloses a thermographic material comprising at least one element and wherein the element(s) contain(s) therein a substantially light-insensitive organic heavy metal salt and an organic reducing agent therefor, the material being capable of thermally producing an image from the organic heavy metal salt and reducing agent, wherein the material contains a 1,3-benzoxazine-2,4-dione toning agent having general formula (I): 
wherein R1 represents hydrogen, xe2x80x94CH2OH, xe2x80x94(Cxe2x95x90O)xe2x80x94R, xe2x80x94CONHR, or M; R2, R3, R4 and R5 each independently represents hydrogen, xe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94OR or xe2x80x94NHxe2x80x94(Cxe2x95x90O)xe2x80x94OR and at least one of which is not hydrogen if R1 is also hydrogen; R represents an alkyl or aryl group either of which may be substituted; and M represents a monovalent heavy metal ion.
EP-A 752 616 specifically discloses substantially light-insensitive thermographic recording materials with a thermosensitive element containing silver behenate and 5, 10 and 20 mol % 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione (compound 1) with respect to silver behenate.
In printing with thermographic materials for medical applications and graphic arts applications it is desirable to increase the throughput of thermographic materials. This requires that thermal development take place over as short a period as possible. In the case of substantially light-insensitive thermographic recording materials in which thermal development is obtained by image-wise heating, this requires that the heating time per pixel be as short as possible. In the case of image-wise heating with the resistance elements of a thermal head, this means that the line time of the thermal head be as short as possible without loss in image tone in continuous tone images. Image tone can be assessed on the basis of the L*, a* and b* CIELAB-values, which are determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90.
It is an object of the present invention to provide a novel tone modifier for use in substantially light-insensitive thermographic recording materials.
It is a further object of the present invention to provide a means of obtaining clinically acceptable image tones in high throughput substantially light-insensitive thermographic recording material for printers with thermal head line times of 20 ms to 4.5 ms or less at a resolution of at least 118 dots per cm (=300 dots per inch).
Further objects and advantages of the invention will become apparent from the description hereinafter.
Three crystalline phases of silver behenate have been identified by X-ray diffraction measurements with a copper Kxcex11 X-ray source, which have been designated: phase I, phase II and phase III. At 25xc2x0 C. pure silver behenate only exists in the well-known crystalline phase designated as phase I [ICDD reference spectrum: 4.53xc2x0, 6.01xc2x0, 7.56xc2x0, 9.12xc2x0, 10.66xc2x0, 12.12xc2x0 and 13.62xc2x0 (National Institute of Standards, Gaithersburg, Md.-20899-0001, USA)] and as an amorphous phase. Phase II silver behenate, a mesomorphous phase having an X-ray diffraction spectrum upon irradiation with a copper Kxcex11 X-ray source with Bragg angles 2"THgr" of 5.34-5.78xc2x0, 6.12-6.41xc2x0, 7.68-7.79xc2x0, 8.30-8.59xc2x0, 9.36-9.84xc2x0, 10.6-10.96xc2x0, and phase III silver behenate, a mesomorphous phase having an X-ray diffraction spectrum upon irradiation with a copper Kxcex11 X-ray source with Bragg angles 2"THgr" of 4.76-4.81xc2x0, 5.9-6.3xc2x0, 6.76-7.35xc2x0, 8.27-8.44xc2x0 and 9.06-9.43xc2x0 are, in the case of pure silver behenate, only stable between 130-140xc2x0 C. and ca. 156xc2x0 C.; and between ca. 156xc2x0 C. and ca. 180xc2x0 C. respectively.
It has been surprisingly found that phase II and phase III are identifiable on the basis of their X-ray diffraction spectra in certain substantially light-insensitive thermographic recording materials after thermographic printing. Furthermore, the presence of these silver behenate phases in the printed thermographic materials could surprisingly be correlated with improved image tone. Therefore, such silver behenate phases, when stabilized at room temperature, act as tone modifiers.
Surprisingly, it has been found in model experiments that phase II and phase III silver behenate, which for pure silver behenate are only stable at temperatures between 130-140 and ca. 156xc2x0 C. and ca. 156xc2x0 C. and ca. 180xc2x0 C. respectively, can be stabilized at 25xc2x0 C. to different degrees by the presence of a compound selected from the group consisting of: glutaric acid, benzo[e][1,3]-oxazine-2,4-dione; substituted benzo[e][1,3]-oxazine-2,4-dione compounds such as 7-(ethylcarbonato)-benzo[e][1,3]-oxazine-2,4-dione, 7-methyl-benzo[e][1,3]-oxazine-2,4-dione and 7-methoxy-benzo[e][1,3]-oxazine-2,4-dione; phthalazinone; and polyvinyl butyral, or combinations thereof. The broadness of the XRD-peaks for these two phases leads to significant XRD-peak overlap between the two phases, when present at the same temperature. This stabilization of phase II and phase III silver behenate is only observable in materials in which all the residual silver behenate is not converted into another silver salt, during the thermal development process.
A possible explanation for the tone modifying effect of phase II and phase III silver behenate is that these phases promote the formation of metallic silver nuclei clusters with a size at which light scattering produces a blue-black neutral image tone [xe2x89xa7160 nm according to Bird in Photographic Science and Engineering, volume 15, page 356 (1971)].
The above-mentioned objects are realized by providing a substantially light-insensitive thermographic recording material comprising a support and a thermosensitive element, the thermosensitive element containing silver behenate including phase I silver behenate having an X-ray diffraction spectrum upon irradiation with a copper Kxcex11 X-ray source with Bragg angles 2"THgr" of 4.53xc2x0, 5.96-6.05xc2x0, 7.46-7.56xc2x0, 8.90-9.12xc2x0, 10.45-10.66xc2x0, 12.02-12.12xc2x0, 13.53-13.62xc2x0, a reducing agent therefor in thermal working relationship therewith and a binder, wherein the thermographic recording material is capable upon thermal development of containing 1% of phase II silver behenate, having an X-ray diffraction spectrum upon irradiation with a copper Kxcex11 X-ray source with Bragg angles 2"THgr" of 5.34-5.78xc2x0, 6.12-6.41xc2x0, 7.68-7.79xc2x0, 8.30-8.59xc2x0, 9.36-9.84xc2x0, 10.6-10.96xc2x0, which is stable at 25xc2x0 C., and/or phase III silver behenate phase, having an X-ray diffraction spectrum upon irradiation with a copper Kxcex11 X-ray source with Bragg angles 2"THgr" of 4.76-4.81xc2x0, 5.9-6.3xc2x0, 6.76-7.35xc2x0, 8.27-8.44xc2x0 and 9.06-9.43xc2x0, which is stable at 25xc2x0 C., with respect to the quantity of the phase I silver behenate in the thermographic recording material before the thermal development.
A recording process is also provided by the present invention for a thermographic recording material, the thermographic recording material comprising a thermosensitive element, the thermosensitive element comprising silver behenate including the above-defined phase I silver behenate, an organic reducing agent therefor in thermal working relationship therewith and a binder, comprising: (i) converting the silver behenate into the above-defined phase II silver behenate and/or the above-defined phase III silver behenate; and (ii) cooling the thermographic recording material to 25xc2x0 C., wherein at least 1% of the phase II silver behenate and/or the phase III silver behenate, with respect to the quantity of the phase I silver behenate in the thermographic recording material before the recording process, is present in the cooled thermally developed thermographic recording material at 25xc2x0 C. as stable phases.
A use is also provided by the present invention of the above-defined phase II silver behenate stabilized at 25xc2x0 C. and/or the above-defined phase III silver behenate defined in claim 1 stabilized at 25xc2x0 C. as a tone modifier in thermographic recording materials.
Preferred embodiments of the present invention are disclosed in the dependent claims.